Marine Benthic Cyanobacteria as Potential Bioresources for Rare Earth Element Management: Insights from Lanthanum Bioaccumulation and Strain-Specific Physiological Responses

Marine cyanobacteria represent a promising but underexplored bioresource for rare earth element (REE) management. This study investigated the effects of lanthanum (La, 10 mg L−1) on four cyanobacterial strains isolated from Black Sea periphyton: Desertifilum tharense AqMaPh-IBSS-CYA-1, Toxifilum mysidocida AqMaPh-IBSS-CYA-10, Salileptolyngbya sp. IBSS-CYA-8, and Cyanobium sp. AqMaPh-IBSS-CYA-15. La induced a delayed hormetic response in Cyanobium sp. Under suboptimal light and temperature conditions, La supplementation supported 100% survival of all strains over 35 days, whereas controls perished. SEM-EDX indicated predominantly intracellular La localization, with surface-bound La detected only sporadically. ICP-MS revealed hyperaccumulation of La (9.5–13.3 mg g−1 dry weight, 110–226-fold enrichment) and co-accumulation of light REEs in filamentous strains. Strain-specific responses suggest distinct protection mechanisms: T. mysidocida may employ metal efflux and polyphosphate granule sequestration, suggesting potential for REE recovery; Salileptolyngbya sp. appears to show broad-spectrum metal binding, potentially useful for bioremediation; D. tharense seems to be the most sensitive under the tested conditions, suggesting potential as a REE bioindicator. These findings are preliminary, but they suggest that La may act not only as a toxicant but also as a metabolic modulator, highlighting the potential of these cyanobacteria as bioresources for sustainable REE management.